In the manufacture of raw metal goods, such as steel wire, steel bars, etc., an oxide film forms on the end product due to the reaction of oxygen with the hot iron after annealing or other heat treatment and prior to quenching. In one significant example, magnetite (Fe.sub.3 O.sub.4) - containing films occur on the surfaces of iron-containing steel goods. The magnetite film is produced at elevated temperatures during extrusion when the metal goods come into physical contact with an oxygen-containing gas, such as air. An elevated temperature is typically required in the case of steel wire, for example, to reduce the stress produced by the extrusion process. This particular oxide, i.e. magnetite, must be removed from the steel wire prior to any subsequent processing such as galvanizing and/or electroplating. The subsequent processing will not be successful unless the underlying wire metal surface is free from magnetite. Hence, the efficient, complete removal of magnetite from the underlying metal product is quite important and highly desirable.
A traditional method of removing such oxide films is to immerse the oxide-coated metal product in an acid bath, referred to generally as acid pickling. Hydrochloric and sulfuric acid are two examples of appropriate acid mediums. Such acids are capable of dissolving ferric oxide (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4) and ferrous oxide (FeO). The acid bath is effective in removing the oxide, producing water and various iron salts in a well known chemical reaction. Following removal of the oxide, the iron in an underlying steel product will continue to dissolve, with hydrogen gas being produced as well as dissolved iron salts.
Although acid pickling produces a rapid removal of the oxide, it does have several disadvantages, including possible hydrogen embrittlement of the underlying metal product, due to the absorption of hydrogen by the metal product, resulting in blisters, cracks and flakes in the metal product. Further, the chemicals used in the pickling process are typically quite corrosive and are usually quite expensive, adding to the cost of the process. Toxic effluents also are produced by the process and they must be treated. Lastly, the concentration of the acid pickling bath continuously changes, due to the change in the amount of dissolved salts produced during the process and in solution in the acid bath.
There have been attempts to remove oxides with methods other than pickling. One such attempt is described in U.S. Pat. No. 4,795,537 to Timewell. While the '537 process is successful in removing many oxides, there are some oxides, such as magnetite, which are resistant to the Timewell pulsating DC electrolysis system as described.
Hence, there is a need for a process to efficiently remove magnetite and similar oxide coatings from metal products, such as wire, metal rods, etc. following the manufacture (extrusion) thereof, in order to permit the use of additional processing such as, for instance, electroplating.